The present invention relates to the production of tubing of composite material particularly adapted for air conditioning systems installed onboard aircraft.
In an aircraft, the air conditioning tubing uses recycled air. This air is cooled by heat exchangers to utilization temperatures comprised between 20° C. and 30° C. In the case of failure of these heat exchangers, the tubing is dimensioned to resist temperatures of the order of 70° C.
Moreover, in the presence of fires, said tubing must be self-extinguishing and cannot give off smoke or toxic gases to be inhaled, so as to preserve the life of the passengers and the crew members.
Finally, this tubing must be sealed, because any loss of air gives rise to the requirement for supplemental air, thereby increasing the consumption of fuel.
At present, this tubing is comprised by two layers of glass cloth impregnated with phenolic resin. The unit weight of one ply is about 300 g/m2, said ply being impregnated with 40% of resin, which brings the unit weight of the ply to 420 g/m2. The use of two superposed plies thereby constituting an assembly of a unit weight of 840 g/m2, permits guaranteeing the structural resistance of the final product.
These two plies are draped in or on a metallic or composite mold having the final shape of the tubing. Two technologies are conventionally used as to the mold, which can be either constituted by two assembleable portions, defining a hollow having the shape of the piece to be produced, on which the plies are disposed, an inflatable bladder being then installed on the plies and then inflated to press them against the mold, or constituted by a dummy with the shape of the piece to be produced, on which said plies are draped, vacuum then being drawn between the plies and a sealing film covering these latter.
When the plies are pressed into or onto the mold, the assembly is installed in an oven or autoclave so as, either to raise the temperature alone, or to raise the temperature and the pressure, for the purpose of polymerization.
Once this first polymerization is completed, and the tubing thus produced returned to ambient temperature, a phenolic resin based varnish is applied with the help of a paint spray gun or a simple brush over all the external portion of said tubing, so as to close porosities that can exist following polymerization.
The use of said varnish causes problems as to hygiene and safety, because of the presence of the solvent contained in the phenolic resin. A second polymerization in an oven is thus carried out so as to polymerize this layer of varnish. Sealing tests (pressure/vacuum) are then carried out so as to verify the sealing of the tubing.
In the case in which sealing is defective, a second layer of varnish is then applied to the tubing, which is then returned to the oven so as again to be polymerized. Again, tests are carried out on said tubing so as to monitor its sealing. If the sealing is again defective, the tubing will be subjected to a new cycle of varnish/polymerization/tests. All the tubing thus produced is tested so as to verify its level of sealing.
Thus, this process of producing tubing is difficult and gives rise to a long production cycle as to the technological time of production of said tubing. It is not consistent with industrial objectives which are the reduction of cycle times and costs. Not only does this process slow the production cycle of the tubing, but also it uses products whose use gives rise to hygienic and safety concerns. Moreover, the tubing thus produced does not conform to the objectives of weight gain and price because two plies of pre-impregnated cloth are used, to which are added the various layers of varnish.